The present invention relates to a composition and method for enhancing the recovery of petroleum from an oil-bearing formation.
In the recovery of oil from reservoirs, the use of primary production techniques (i.e., the use of only the initial formation energy to recover the crude oil) followed by the secondary technique of waterflooding, recovers only a portion of the original oil present in the formation.
Moreover, the use of certain enhanced oil recovery (EOR) techniques is also known in the art. These techniques can generally be classified as either a thermally based recovery technique, e.g., utilizing steam, or a gas drive method that can be operated under miscible or non-miscible conditions.
The gases which are commonly employed in gas-drive methods include for example, nitrogen, carbon dioxide, methane, mixtures of methane with ethane, propane, butane, and higher hydrocarbon homologues. This class of gases includes both natural gas and produced gas.
A typical procedure involves injecting a slug of CO.sub.2 followed by the injection of a higher viscosity fluid such as water to "push" the CO.sub.2. See, for example, the discussion in U.S. Pat. No. 2,623,596. Moreover, U.S. Pat. No. 3,065,790 indicates that this process may be more cost effectively employed if the slug of CO.sub.2 is relatively small. In fact, as illustrated by U.S. Pat. No. 3,529,668, this type of recovery procedure is typically performed in "water alternating gas (WAG)" cycles.
Because of the viscosity and density differences between the CO.sub.2 and the oil (i.e., viscosity of CO.sub.2 is only 5 to 10% of the viscosity of light oil), the C.sub.2 tends to bypass much of the oil when flowing through the pores of the reservoir rock.
One proposed solution to this problem associated with the bypassing of the CO.sub.2 includes the injection of water which contains a surfactant, with the CO.sub.2. In particular, surfactants have been proposed as a means for generating a foam or an emulsion in the formation. See, for example, U.S. Pat. No. 4,380,266 to Wellington, U.S. Pat. No. 4,860,828 to Oswald et al and U.S. Pat. No. 5,502,538 to Wellington et al.
The purpose of this foam is to divert the flow of the C.sub.2 into that portion of the formation containing high oil saturation.
One surfactant composition includes alpha-olefin sulfonate (AOS) surfactants and in particular C.sub.12 AOS, i.e., an AOS having a carbon chain length of 12.
Furthermore, although AOS surfactants are largely known as "good foamers", the creation of an effective foam requires that the surfactant be completely soluble in the injection water. Solubility is limited when employing AOS having longer chain lengths, i.e., greater than about C.sub.10, by the amount of salt in the injection water or formation brine. This is a particular problem when the salt concentration is above about 15 wt %.
In such environments, the AOS can separate out and become adsorbed onto the formation rock. Thus, any foam which is formed is not effectively propagated through the reservoir.
In order to solve problems associated with brine tolerance, a variety of materials are recognized in the art as being effective "solubilizers", i.e., compounds which are not suitable foaming agents but which can improve brine tolerance of less brine tolerant materials. Materials which have been employed as solubilizers include nonionic surfactants such as ethoxylated nonylphenols and secondary alcohols, ethylene/propylene oxide copolymers, fatty acid ethanolamides, glycols, and polyglycosides as well as certain anionic, cationic and amphoteric surfactants and certain sequesterants. See for example, the discussion in McCutcheon's Functional Materials, pp 220-230, 1989.
However, these materials have not proven effective with C.sub.10-16 AOS surfactants, particularly when employed in brines at high temperatures, i.e., above about 100.degree. F. Accordingly, the need still exists for a foam-forming composition comprising C.sub.10-16 AOS surfactants which has improved brine tolerance.
These and further aspects will become apparent from the specification and drawings which follow.